Numerical simulation of sheet metal forming processes and localized deformation phenomena for FCC polycrystals

Une formulation par éléments finis pour les polycristaux, basée sur l'hypothèse de Taylor pour un matériau sensible au taux de déformation, a été appliquée pour analyser la mise en forme des métaux et les phénomènes de localisation. Cette formulation s'applique aux problèmes de conditions aux limites non homogènes des métaux polycristallins sous grandes déformations. Elle tient également compte des textures initiales ainsi que des anisotropies induites par la déformation causée par l'évolution de la texture. Des codes d'éléments finis bases sur les hypothèses de déformations planes et de contraintes planes incorporant des algorithmes de calcul parallèle ont été élaborés. En utilisant les codes d'éléments finis établis, les instabilités et les phénomènes de localisation dans l'alliage d'aluminium AA3004-H19 sous tension ont alors été étudiés. Les effets de divers paramètres tels que la texture initiale et son évolution, l'écrouissage, la sensibilité au taux de déformation du matériau, la direction du chargement, la sensibilité du maillage, les imperfections géométriques et les conditions limites sur la formation des déformations locales ont été traités. Un critère d'instabilité a été défini pour la striction et la formation des bandes de cisaillement. Le comportement à grandes déformations pour l'alliage d'aluminium AA3004-H19, sous cisaillement simple, a aussi été simule numériquement en utilisant les codes d'éléments finis. Les effets des directions du cisaillement sur les courbes de contrainte-déformation et les morphologies des déformations ont été étudiés. L'initiation et la propagation des bandes de cisaillement ont aussi été examinées en détail. Finalement, l'emboutissage profond des alliages d'aluminium AA6111-T4 et AA5754-0 a été modélisé en utilisant à la fois Ie modèle polycristallin et un modèle phénoménologique où seulement la zone de « flange » a été analysée. Les effets de ces textures ont été examines et les résultats obtenus ont été comparés avec les données expérimentales.Abstract: In this thesis, finite element analyses based on a rate-dependent Taylor-type polycrystal model have been developed to simulate sheet metal forming processes and localized deformation phenomena. This formulation can be applied to nonhomogeneous boundary-value problems for FCC polycrystals subjected to large deformations. The analysis inherently accounts for initial textures as well as deformation-induced anisotropies due to texture evolution. Both plane strain and plane stress finite element (FE) codes incorporating parallel computing algorithms have been developed so that simulations could be performed for applications requiring fairly large numbers of elements. Using the finite element codes which have been developed, instability and localization phenomena for the rolled aluminum sheet alloy AA3004-H19 under tension have been studied. The effects of various parameters on the formation of localized deformation bands have been investigated. These include initial texture and its evolution, strain hardening, material strain-rate sensitivity, loading direction, mesh sensitivity, geometric imperfections, and boundary conditions. Instability criteria have been defined for both necking and shear banding. The large strain behaviour of the rolled aluminum sheet alloy AA3004-H19 under planar simple shear has also been simulated numerically using both the plane strain and the plane stress polycrystal FE codes. The effects of the shearing direction on the overall shear stress--shear deformation curves and deformation patterns have been investigated. The initiation and propagation of shear bands have been studied in detail. Finally, the plane strain FE code was employed to simulate earing during the deep drawing of the rolled aluminum sheet alloys AA6111-T4 and AA5754-0. Simulations based on both the polycrystal model and a phenomenological constitutive law were performed where only the flange area of the sheet was analyzed. The effects of these textures were examined, and comparisons were made with experimental data

Average glandular doses and national diagnostic reference levels in mammography examinations in Turkey

In order to establish national diagnostic reference levels for mammography examinations, entrance skin air kerma, entrance skin dose and average glandular doses (AGDs) were calculated for a total of 25 624 cranio-caudal (CC) and mediolateral oblique (MLO) projections of 6309 patients for 40-49 and 50-64 age groups. The average entrance skin air kerma and entrance skin dose values for both age groups were found to be higher in MLO projections compared with CC projections. The minimum and maximum values of AGDs were determined as 0.4 and 7.9 mGy for both projections. The maximum numbers of AGDs for CC and MLO projections were calculated in the range of 1.1-1.5 and 1.6-2.0 mGy, respectively. The third quartile values of AGDs were calculated for each compressed breast thickness between 20 and 99 mm. The first national diagnostic reference levels of the country were established for each 10-mm compressed breast thickness in mammography examinations.Turkish Atomic Energy AuthorityMinistry of Energy & Natural Resources - Turke

Unintentional irradiation of conceptus by diagnostic imaging examinations in Turkey

Exposure of the fetus to medical radiation sources during the diagnostic procedures without intention is one of the most significant concerns in the medical community. In this study, 45 conventional X-ray and computed tomography (CT) examinations of the women who were unaware of their pregnancy were investigated. Effective doses and fetal doses were calculated for each application by using PCXMC and ImPACT CT scan software. The exposure of abdominal CT and abdominal conventional X-ray examinations was found to be over the literature for both the range and the average values. Average effective dose for abdominal CT examinations was calculated to be similar to 3.1 times higher than that in the literature. For abdominal CT and conventional X-ray examinations, the mean fetal doses were found to be similar to 3.5 times and similar to 5.4 times higher than those in the literature, respectively.Turkish Atomic Energy AuthorityMinistry of Energy & Natural Resources - Turke

Novel thresholding method and convolutional neural network for fiber volume content determination from 3D μCT images

In order to determine fiber volume contents (FVC) of low contrast CT images of carbon fiber reinforced polyamide 6, a novel thresholding method and a convolutional neural network are implemented with absolute deviations from experimental values of 2.7% and, respectively, 1.46% on average. The first method is a sample thickness based adjustment of the Otsu threshold, the so-called “average or above (AOA) thresholding”, and the second is a mixed convolutional neural network (CNN) that directly takes 3D scans and the experimentally determined FVC values as input. However, the methods are limited to the specific material combination, process-dependent microstructure and scan quality but could be further developed for different material types

Deep convolutional generative adversarial network for generation of computed tomography images of discontinuously carbon fiber reinforced polymer microstructures

Computed tomography images are of utmost importance when characterizing the heterogeneous and complex microstructure of discontinuously fiber reinforced polymers. However, the devices are expensive and the scans are time- and energy-intensive. Through recent advances in generative adversarial networks, the instantaneous generation of endless numbers of images that are representative of the input images and hold physical significance becomes possible. Hence, this work presents a deep convolutional generative adversarial network trained on approximately 30,000 input images from carbon fiber reinforced polyamide 6 computed tomography scans. The challenge lies in the low contrast between the two constituents caused by the close proximity of the density of polyamide 6 and carbon fibers as well as the small fiber diameter compared to the necessary resolution of the images. In addition, the stochastic, heterogeneous microstructure does not follow any logical or predictable rules exacerbating their generation. The quality of the images generated by the trained network of 256 pixel x 256 pixel was investigated through the Fréchet inception distance and nearest neighbor considerations based on Euclidean distance and structural similarity index measure. Additional visual qualitative assessment ensured the realistic depiction of the complex mixed single fiber and fiber bundle structure alongside flow-related physically feasible positioning of the fibers in the polymer. The authors foresee additionally huge potential in creating three-dimensional representative volume elements typically used in composites homogenization

Efficient fast Fourier transform-based numerical implementation to simulate large strain behavior of polycrystalline materials

The final publication is available at Elsevier via https://doi.org/10.1016/j.ijplas.2017.07.001 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/In this paper, a new full-field numerical framework is proposed to model large strain phenomena in polycrystals. The proposed framework is based on the elasto-viscoplastic (EVP) fast Fourier transform (FFT) formulation presented by Lebensohn et al. (2012) and the rate dependent crystal plasticity framework developed by Asaro and Needleman (1985). In this implementation, the full-field solutions of micromechanical fields are computed on a regular, voxelized representative volume element (RVE) in which either a single or multiple grid points represent a single grain. The Asaro and Needleman (1985) formulation coupled with a semi-explicit, forward gradient time-integration scheme (Peirce et al., 1983) is used to compute local stresses and the FFT-based method is used to find local strain fluctuations at each grid point. The proposed model is calibrated using experimental uniaxial tensile test results of aluminum alloy (AA) 5754 sheet and then used to predict texture evolution and stress-strain response for balanced biaxial tension and plane-strain tension along rolling (RD) and transverse (TD) directions. The predicted stress-strain and texture results show a good agreement with experimental measurements. The CPU time required by the proposed model is compared with the original EVP-FFT model for two separate cases and the proposed model showed significant improvement in computation time (approximately 100 times faster).Natural Sciences and Engineering Research Council of Canada (NSERC) || 441668-1

Patient doses from CT examinations in Turkey

[No Abstract Available

A new crystal plasticity framework to simulate the large strain behaviour of aluminum alloys at warm temperatures

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.msea.2018.04.020 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/To improve metal formability, high temperature forming has become a desired manufacturing process. Phenomenological plasticity models are widely used in this application, however lack good predictive capability concerning microstructure evolution during forming. Many crystal plasticity hardening models have been developed to predict deformation phenomena of metals during high temperature forming; however, few have thermodynamic self-hardening formulations based on deformation mechanisms. This work presents a crystal plasticity based analysis with a Taylor polycrystal averaging scheme of warm forming employing a new microstructure and dislocation based strain hardening model to simulate deformation behaviour. The hardening model is derived from energy balance between dislocation storage, dislocation accumulation, and dislocation recovery, based on remobilization of immobile dislocations, due to thermal activation. The constitutive formulation is extended to include alloying effects due to solute strengthening of Mg. The material hardening properties of AA5754 are characterized for a range of temperatures at constant strain-rates. A formulation for the kinematics of dynamic strain aging is presented and employed for room-temperature simulations. The hardening characterization is then used to predict stress-strain behaviour of AA5182 for similar conditions. The model shows excellent predictability of experimental results. An analysis on the microstructural connection between temperature and stress-strain response is presented.Canada (NSERC) [no. APCPJ 441668-12]General Motors of Canad

Modélisation numérique de l'aluminium à grandes déformations plastiques : applications au calcul parallèle

Ce travail, réalisé dans le cadre d'un projet de recherche qui implique le Centre de recherche et développement ALCAN à Kingston et notre groupe de mécanique des solides à l'Université de Sherbrooke, a pour objectif principal de modéliser numériquement le comportement de l'aluminium à grandes déformations plastiques. Les modélisations numériques des échantillons cruciformes, l'emboutissage profond et la courbe limite de formage sont réalisés en incorporant les algorithmes de calculs parallèles. Deux codes d'éléments finis sont développés; l'un basé sur les lois phénoménologiques de plasticité et l'autre basé sur la plasticité des cristaux. Les modèles de BARLAT (1989) et HILL (1948 et 1990) sont utilisés dans le modèle basé sur les lois phénoménologiques de plasticité pour tenir compte de l'anisotropie. Le modèle polycristallin qui tient compte du glissement dans certains plans atomiques et de l'anisotropie élastique de la maille cristalline est basé sur les hypothèses de TAYLOR (1923). Les algorithmes de calculs parallèles sont développés pour les applications mentionnées ci-dessus. Des gains de temps importants sont obtenus à l'exécution par la parallélisation de ces applications, en particulier les programmes basés sur la plasticité des cristaux. La modélisation de l'emboutissage profond avec le modèle polycristallin requiert un certain nombre de mégaoctets de mémoire pour traiter des problèmes de grandeurs représentatives. Il est par conséquent indispensable de se servir d'un ordinateur parallèle pour résoudre ce type de problèmes